In the field of short- to medium-haul aircraft, and aviation in general, optimizing fuel use is a factor essential to profitability.
Conventionally, such aircraft are equipped with turbojets which are a compromise between energy cost and the impact on the environment. In fact, other propulsion systems, such as propulsion engines with contrarotating propellers, allow a 20% fuel economy, but are often disregarded due to the unwanted noise they cause. For this type of motorization, the sources of noise to be considered are:                the noise of each propeller itself, due to the pass frequency of the blades and their harmonics, which is a noise of low-frequency type and as a result difficult to mask and        the interaction noise between the propellers.        
The applicant has already noted that each propeller's own noise could be reduced, so that the preponderant noise is that resulting from the interaction between the propellers. He has also been able to determine, by experimental and numerical routes, the source zones of the noise for a turboprop engine with contrarotating propellers which can be modelled by:                a first conical lobe directed forwards from said turboprop engine and centred on the axis thereof, its peak being located approximately midway between the planes of the propellers;        a second conical lobe directed aft from said turboprop engine and centred on the axis thereof, its peak being approximately midway between the planes of the propellers;Thus, in patent application FR 2 905 356, the Applicant disclosed the benefit of masking the upstream noise lobe by the aerofoil and presented several solutions, in particular:        simply moving the aerofoil aft;        adding a supplementary aerofoil surface called a noseplane in order to compensate for the reduction in the handling quality due to moving the aerofoil aft;        using a forward-swept aerofoil in order to reduce the extent of aft movement of the aerodynamic centre compared to a conventional sweep aerofoil.        